In a semiconductor device used in a VLSI of recent years, a severe demand in fine processing rises according to development of its high integration and high performance. Taking the structure of a DRAM as an example, the width of wiring is reduced with the distance of wiring being reduced, and the hole diameter of a contact hole also becomes small. As a result, the distance between the wiring and the contact hole becomes small, and there arises a fear of electric short circuit. In order to prevent the same, a layer of silicon nitride is inserted in addition to an interlayer insulating film formed with an oxide film.
FIG. 1 is a schematic cross sectional view of a DRAM of a COB structure for describing the problems of the conventional dry etching method.
A bit line 106 is formed on a silicon substrate 107, and an oxide film 103 is formed on the bit line 106. A silicon nitride film 104 is formed on the oxide film 103, and an oxide film 103 is formed on the silicon nitride film 104. A word line 105 is formed inside the oxide film 103. A silicon nitride film 104 is formed on the oxide film 103, and an oxide film 103 is formed on the silicon nitride film 104. A silicon nitride film 102 is formed on the oxide film 103, and a capacitor part 101 is formed on the silicon nitride film 102. A contact hole 108 is opened from the capacitor part 101 to a transistor at the lower part of the figure.
In order to produce the contact hole 108, a laminated film composed of the oxide film 103 and the silicon nitride films 102 and 104 should be etched as shown in FIG. 1.
As an etching gas that can etch both the oxide film and the silicon nitride film, a CHF.sub.3 series gas can be exemplified. As an example of fine processing technique in recent years, processing of a contact hole using a polymask instead of a resist mask is being employed. Submicron processing, which has not been accomplished by the resist mask, can be realized by using the polymask.
However, when a contact hole is produced with the polymask by using the CHF.sub.3 series gas singly, there arises a phenomenon in that the selective ratio of the mask and the oxide film to shift the mask.
Furthermore, the conventional etching method involves the following problems.
FIGS. 2 to 4 are schematic cross sectional views showing a part of a production process of a semiconductor device using the conventional etching method, and also describing the problems associated with the conventional etching method.
As shown in FIG. 2, a silicon nitride film 204 is formed on a silicon substrate 205, and an oxide film 203 is formed on the silicon nitride film 204. An etching mask (poly-Si) 201 is formed on the oxide film 203. When a contact hole 202 is formed in the oxide film 203 and the silicon nitride film 204 by etching with the etching mask 201 as a mask by using a CHF.sub.3 series gas singly, the shape of the contact hole becomes a bowing shape.
Thereafter, a hole filler 207, such as poly-Si, is accumulated on the poly-Si (etching mask) 201 to bury the contact hole 202 as shown in FIG. 3. A hollow space 206 is formed inside the contact hole 202 since the contact hole 202 has the bowing shape.
The hole filler 207 is then subjected to etch back. The hollow part 206 is etched at a faster rate than the other part as shown in FIG. 4, and there arises a problem in that the silicon substrate 205 at the bottom of the contact hole 202 is etched, which is not planned to be etched.
As a method for preventing such a problem, a method is considered in that after etching the oxide film 203 with a C.sub.4 F.sub.8 series gas, the silicon nitride film 204 is etched with a CHF.sub.3 series gas. The oxide film is easily etched with the C.sub.4 F.sub.8 series gas, but the silicon nitride film is not easily etched by that gas. In order to practice such a method, after etching the oxide film 203 with the C.sub.4 F.sub.8 series gas, a fluorocarbon series reaction product deposited inside the contact hole must be removed with an O.sub.2 plasma (ashing), and then further cleaned with sulfuric acid and aqueous hydrogen peroxide, followed by etching the silicon nitride film 204 by using the CHF.sub.3 series gas. In the case where the multi-layer film comprising plural oxide films and silicon nitride films is produced as shown in FIG. 1, such a method requires the removing step of the reaction product and cleaning step for each films, to increase the cost.
In order to suppress the cost, on the other hand, a method is considered in that the removing step of the reaction product and the cleaning step are omitted, and after etching the oxide film 203 with a C.sub.4 F.sub.8 series gas, the etching gas is switched from the C.sub.4 F.sub.8 series gas to a CHF.sub.3 gas, to continuously etch the silicon nitride film 204. However, as shown in FIG. 5, the etching rate (etching amount) of the silicon nitride film under the oxide film is decreased in proportion to the over-etching amount of the oxide film with the C.sub.4 F.sub.8 series gas, and when the over-etching amount reaches a specific value, an etching stop phenomenon occurs. Therefore, the removing step of the reaction product and the cleaning step cannot be omitted.
FIGS. 6A, 6B, and 6C are schematic cross sectional view in FIG. 5 showing the phenomenon in that etching stop occurs when the over-etching amount of the oxide film is increased.
As shown in FIG. 6A, a silicon nitride film 304 is formed on a silicon substrate 305, and an oxide film 303 is formed on the silicon nitride film 304. An etching mask 301 is formed on the oxide film 303. The oxide film 303 is then etched with the etching mask 301 as a mask by using a C.sub.4 F.sub.8 series gas 306 to immediately before exposing the surface of the silicon nitride film 304. In this case, no reaction product is formed inside a contact hole 302.
As shown in FIG. 6B, a silicon nitride film 304 is formed on a silicon substrate 305, and an oxide film 303 is formed on the silicon nitride film 304. An etching mask 301 is formed on the oxide film 303. The oxide film 303 is then etched with the etching mask 301 as a mask by using a C.sub.4 F.sub.8 series gas 306 to immediately before exposing the surface of the silicon nitride film 304. In this case, a fluorocarbon series reaction product 307 is formed inside a contact hole 302.
As shown in FIG. 6C, a silicon nitride film 304 is formed on a silicon substrate 305, and an oxide film 303 is formed on the silicon nitride film 304. An etching mask 301 is formed on the oxide film 303. The oxide film 303 is then over-etched with the etching mask 301 as a mask by using a C.sub.4 F.sub.8 series gas 306. In this case, the amount of a fluorocarbon series reaction product 307 formed inside a contact hole 302 is larger than the case of FIG. 6B.
It is understood from these figures that when the oxide film 303 is etched with a C.sub.4 F.sub.8 series gas, the fluorocarbon series reaction product 307 starts to be accumulated inside the contact hole 302 on exposing the silicon nitride film 304 as an underlayer. The amount of the reaction product depends on the over-etching amount of the oxide film 303 with a C.sub.4 F.sub.8 series gas as expected from FIG. 5. When the over-etching time is further prolonged, the etching effect of the ion is cancelled by the reaction product accumulated inside the contact hole, and the etching is stopped. Therefore, after etching the oxide film 303, the fluorocarbon series reaction product 307 is evaporated by ashing with oxygen in the form of COF as a reaction product of O.sub.2 and CF, and then the silicon nitride film 304 is etched.
On the other hand, as a method of removing the reaction product inside the contact hole only by switching the gas conditions in the same etching apparatus, a method is considered in that after completing the etching of the oxide film 203 with a C.sub.4 F.sub.8 series gas, oxygen is introduced into a chamber to remove the reaction product inside the hole. However, in the case where this method is employed, the reaction product attached to the inner wall of the chamber is also removed, which becomes a cause of formation of particles.
In the case where after completing the etching of the oxide film, the reaction product inside the hole is removed by generating an oxygen plasma in the chamber, the plasma invades the back surface of a wafer. At this time, if an electrostatic chuck is used as means for transferring the temperature of a lower electrode, the temperature of which is controlled as a cooling mechanism for the wafer, and a polyimide resin is used as a dielectric film, the polyimide film is also etched to bring about a severe trouble of insulation breakage.